Metallocene catalysts for olefin polymerization have been developed for a long time. Metallocene compounds are generally activated by aluminoxane, borane, borate or other activators and used. For example, aluminoxane is used as an activator for a metallocene compound having a ligand including a cyclopentadienyl group and two sigma chloride ligands. A case in which, when a chloride group of the metallocene compound is replaced with another ligand (e.g., a benzyl or trimethylsilyl methyl group (—CH2SiMe3)), the effect of increasing catalytic activity or the like is exhibited, has been reported.
[Me2Si(Me4C5)NtBu]TiCl2 (constrained-geometry catalyst, CGC) was disclosed by U.S. Pat. No. 5,064,802 of the Dow Co. in the early 1990s, and excellent aspects of the CGC in the copolymerization reaction of ethylene and an alpha-olefin may be summarized in the following two points when compared to known metallocene catalysts: (1) Even at a high polymerization temperature, high activity is shown and a polymer having a high molecular weight is produced, and (2) the copolymerization degree of an alpha-olefin having large steric hindrance such as 1-hexene and 1-octene is excellent.
Further, as various properties of the CGC during a polymerization reaction are gradually known, efforts of synthesizing the derivatives thereof and using them as a polymerization catalyst have been actively conducted in academia and industry.
As one approach, the synthesis of a metal compound introducing various bridges instead of a silicon bridge and a nitrogen substituent and the polymerization thereof has been conducted. In representative metal compounds known until now, phosphorous, ethylene or propylene, methylidene or methylene bridges are introduced instead of the silicon bridge of a CGC structure, but excellent results in terms of polymerization activity, copolymerization performance or the like could not be obtained by applying to ethylene polymerization or copolymerization with an alpha-olefin when compared to those obtained by applying the CGC.
As another approach, many compounds including an oxido ligand instead of the amido ligand of the CGC have been synthesized, and attempts at polymerization using them have been conducted to some extent.
Further, a variety of asymmetric non-crosslinked metallocenes have been developed. For example, (cyclopentadienyl)(indenyl) and (cyclopentadienyl)(fluorenyl) metallocene, (substituted indenyl)(cyclopentadienyl) metallocene and the like are known.
However, in view of commercial use, the catalyst compositions of the non-crosslinked metallocenes do not sufficiently exhibit the polymerization activity of olefins, and polymerization of a polyolefin having a high molecular weight is difficult.